Patent GB-653423-A (GB)
Title: Improvements in or relating to telephone systems
Inventor: PEARCE OWEN AVIS
IPC: H04Q3/62
Filed: 1948-11-06
Abstract: 653,423. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Nov. 6, 1948, No. 28897. [Class 40 (iv)] On receiving a malicious call, the called subscriber in a last party release system may by dialling a high value digit, signal an exchange attendant who is provided with equipment which automatically breaks in to the established connection and gives a visual indication of the calling subscriber's number. As described in a 600-line P.A.X., privileged subscribers are enabled to break in on busy connections. The final selector, Figs. 1, 2, may be seized over either leads 1 or 2 from separate levels of the preceding group selector. In the latter case wiper switching relay WS pulls up after the final selector has been set to the correct level. Breaking in. Privileged subscribers are provided with a lead M earthed over a high resistance. When such a subscriber encounters a busy condition, he dials " 0 " and the dialling relay C completes a circuit from lead M to charge a condenser QA. On the release of the dialling relay the condenser QA is connected to relay CR which operates and locks. Relay DA switches through to the busy connection, and relay E removes busy tone. The wanted party is then requested to release, whereupon relay H operates and DA and E fall back to connect up ringing current. Malicious calls. When the caller hangs up, relay A alone releases, for guard relay B is maintained over an alternative circuit dependent on the operation of DA, the latter being a slave of relay D which is held over the called party's loop. On the called party dialling a high value digit, the operation of the dialling relay C charges the condenser QA over lead CH from earth in a common control circuit CC, Fig. 3. The subsequent release of C connects the condenser to relay CR, which locks up and completes a circuit for the start relay ST of the control circuit and connects the p-wire to earth over a low resistance test relay FT and prevents the charging of any other condenser QA by connecting battery to lead CH. Relay ST starts a switch TF, the outlets of which are connected to the twentyfive primary trunks (i.e. the trunks between the subscribers uniselectors and the group selectors), to hunt for the marked p-wire and, when it is successful, relay FT pulls up and K follows to light a lamp LP2 and to sound an audible alarm. The attendant throws key KS and can then speak to the called party. To ascertain the number of the malicious caller, the attendant throws key KCLD, thus connecting positive marking battery over lead PM to the caller's p-wire and completing circuits for the start relays SA of six access circuits AC, Fig. 3, each having an eightbank switch CL. Banks CL1, 2 ; 3, 6, of each switch are paired and have outlets to the p-wires of a group of 100 subscribers and the remaining banks are connected to the tens and units lamps TL, UL, Fig. 4, respectively, of a number indicator. When one of the switches CL finds the marked calling line, one of test relays FTU, FTO operates, e.g. FTU, and KU, KN follow to light the relevant hundreds lamp HL and the tens and units lamps. Relay CO operates to stop the unsuccessful switches CL and to release the start relays SA. The connection is released by the attendant momentarily throwing key KR, whereupon K falls back and lamp LP2 is extinguished.